DG intentional islanding using padmount transformer interrupters

ABSTRACT

A system and method for allowing islanding in an underground power distribution network when power is lost to the network. The network includes transformers having source side and load side switching devices and servicing a transformer section. The method includes detecting loss of voltage to the network, opening the source side and the load side switching devices in all of the transformers and allowing distributed power generation sources in the sections serviced by the transformers to provide power to the section. The method also includes closing one or both of the source side switching device and the load side switching device of a transformer that services a section that has power generation sources providing excess power, and closing a source side switching device or a load side switching device in an adjacent transformer to the transformer section providing excess power to provide power to the adjacent section.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority from the U.S.Provisional Application No. 63/115,821, filed on Nov. 19, 2020, thedisclosure of which is hereby expressly incorporated herein by referencefor all purposes.

BACKGROUND Field

The present disclosure relates generally to a system and method forallowing islanding in an underground power distribution network whenpower is lost to the network and, more particularly, to a system andmethod for allowing islanding in an underground power distributionnetwork when power is lost to the network by systematically operatingswitches at adjacent transformer sections.

Discussion of the Related Art

An electrical power distribution network, often referred to as anelectrical grid, typically includes a number of power generation plantseach having a number of power generators, such as gas turbines, nuclearreactors, coal-fired generators, hydro-electric dams, etc. The powerplants provide power at a variety of medium voltages that are thenstepped up by transformers to a high voltage AC signal to be connectedto high voltage transmission lines that deliver electrical power to anumber of substations typically located within a community, where thevoltage is stepped down to a medium voltage for distribution. Thesubstations provide the medium voltage power to a number of three-phasefeeders including three single-phase feeder lines that carry the samecurrent, but are 120° apart in phase. A number of three-phase and singlephase lateral lines are tapped off of the feeder that provide the mediumvoltage to various distribution transformers, where the voltage isstepped down to a low voltage and is provided to a number of loads, suchas homes, businesses, etc.

Some power distribution networks may employ a number of undergroundsingle-phase lateral circuits that feed residential and commercialcustomers. Often times these circuits are configured in a loop and fedfrom power sources at both ends, where an open circuit location,typically at a transformer, is used in the circuit to isolate the twopower sources. These residential loop circuits sometimes include homesor businesses that have power generation capabilities, for example,through generators, photovoltaic cells, wind turbines, battery modules,etc., known generally as distributed power generation, whereself-powered individual homes, groups of homes or businesses that arenot connected to the grid is often referred to as islanding. The powerservice provider or utility generally requires that their customers thathave distributed power generation capability agree not to inject poweronto the circuit from their source, known generally as non-exporttariffs. This is important because if a utility worker is dispatched tothe loop circuit for service or repair, that person may not know thatpower is being injected onto the circuit by the distributed powersources, thus creating a possible safety issue. Exporting power across atransformer causes an increase in voltage experienced by other customersbeing serviced by the same transformer, which can result in voltage thatexceeds the maximum voltage that the utility is required to regulate.Therefore, certain equipment and schemes are often employed by utilitiesto prevent islanding of distributed power generation and for enforcingnon-export tariffs.

SUMMARY

The following discussion discloses and describes a system and method forallowing islanding in an underground power distribution network whenpower is lost to the network. The network includes a power line, aplurality of transformers electrically coupled to and positioned alongthe power line, where each transformer services a section of the networkhaving loads and includes a source side switching device and a load sideswitching device, and where source power is provided to both ends of thepower line and one of the switching devices is a normally open switchingdevice. The method includes detecting loss of voltage to the network,opening the source side and load side switching devices in all of thetransformers and allowing distributed power generation sources in thesections serviced by the transformers to provide power to the section.The method also includes closing one or both of the source sideswitching device and the load side switching device of a transformerthat services a section that has power generation sources providingexcess power, and closing a source side switching device or a load sideswitching device in an adjacent transformer to the transformer sectionproviding excess power to provide power to the adjacent section.

Additional features of the disclosure will become apparent from thefollowing description and appended claims, taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of a residential powerdistribution network including transformers having a pair of switchingdevices;

FIG. 2 is a schematic block diagram of one of the transformers in thenetwork shown in FIG. 1 ;

FIG. 3 is an isometric view of one of the transformers in the networkshown in FIG. 1 ; and

FIG. 4 is a cross-sectional type view of a switching device used in thetransformer shown in FIG. 3 .

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the disclosure directedto a system and method for allowing islanding in an underground powerdistribution network when power is lost to the network is merelyexemplary in nature, and is in no way intended to limit the invention orits applications or uses. For example, as mentioned, the system andmethod have particular application for use in a residential loopcircuit. However, the system and method may have other applications.

FIG. 1 is a simplified schematic diagram of an underground powerdistribution network 10 that is fed power from both ends. The network 10includes, for example, two single-phase, self-powered, magneticallyactuated reclosers 12 and 14 connected to the same or different feeders(not shown), or other source of power, at the head ends of the network10, an underground distribution power line 16 and ten transformers 18,20, 22, 24, 26, 28, 30, 32, 34 and 36 coupled along the line 16. Eachtransformer 18, 20, 22, 24, 26, 28, 30, 32, 34 and 36 includes a sourceside towards the source of power and a load side towards a normally openpoint (58 below) in the line 16. The transformer 18 includes source sideand load side switching devices 40 and 42, respectively, the transformer20 includes source side and load side switching devices 44 and 46,respectively, the transformer 22 includes source side and load sideswitching devices 48 and 50, respectively, the transformer 24 includessource side and load side switching devices 52 and 54, respectively, thetransformer 26 includes source side and load side switching devices 56and 58, respectively, the transformer 28 includes source side and loadside switching devices 60 and 62, respectively, the transformer 30includes source side and load side switching devices 64 and 66,respectively, the transformer 32 includes source side and load sideswitching devices 68 and 70, respectively, the transformer 34 includessource side and load side switching devices 72 and 74, respectively, andthe transformer 36 includes source side and load side switching devices76 and 78, respectively. The switching device 58 is normally open toprovide electrical isolation between the source ends of the network 10.

The switching devices 40-78 can be any switching device suitable for thepurposes discussed herein. The switching devices 40-78 can operate asfault interrupting devices or as sectionalizers. As used herein,sectionalizers detect overcurrent, but do not provide fault interruptingor reclosing, increase a count each time they detect loss of voltageduring a fault clearing operation, and lock open if their count hasreached a predetermined value and no current is flowing through thedevice in response to receiving a message.

The transformer 20 is shown with a primary coil 80 electrically coupledto the line 16 between the switching devices 44 and 46, a secondary coil82 electrically coupled to the primary coil 80 and a service conductor84 electrically coupled to the secondary coil 82 and providing power toa number loads 86, i.e., houses, where the other transformers 18 and22-36 would be similarly configured with these same elements. One ormore of the loads 86 may have its own power source 88, such as agenerator, photovoltaic cells, a wind turbine, battery modules, etc.,where the power source 88 would include an inverter 98 for converting DCto AC in a manner well understood by those skilled in the art. Exportingpower occurs when the instantaneous distributed power generated at thepower source 88 exceeds the instantaneous power required by the load 86.

FIG. 2 is a simple schematic block diagram of the transformer 18 withthe understanding that the other transformers 20, 22, 24, 26, 28, 30,32, 34 and 36 are the same or similar. The devices 40 and 42 share acommon control unit 90 and one or more capacitors 92 are coupled betweenthe line 16 and the control unit 90 on the source side and one or morecapacitors 94 are coupled between the line 16 and the control unit 90 onthe load side for measuring voltage on the source and load sides of thetransformer 18. Particularly, the capacitors 92 measure voltage on thesource side of the transformer 18, the capacitors 94 measure voltage onthe load side of the transformer 18 and the control unit 90 measuresvoltage on a transformer bus 96 between the switching devices 40 and 42.The capacitors 92 would be physically located within the device 40 andthe capacitors 94 would be physically located within the device 42.

FIG. 3 is an isometric view of the transformer 18, which is the typethat is mounted on a pad (not shown) with the understanding that thetransformers 20, 22, 24, 26, 28, 30, 32, 34 and 36 are the same orsimilar. It is noted that the configuration of the transformer 18 ismerely for illustrative purposes in that other configurations would beapplicable consistent with the discussion herein. The transformer 18includes an enclosure 100 that houses the coils 80 and 82 and otherelectrical components (not shown) of the transformer 18. A cover 102 ofthe enclosure 100 is shown in an open position to expose a panel 104 inthe enclosure 100. An elbow connector 106 is connected to the line 16and a load-break interface 108 of the switching device 40. A transformerinterface 118 of the switching device 40 is connected to a connectorbushing 120 that extends through the panel 104 and is coupled to theprimary coil 80 to connect the line 16 to the primary coil 80 throughthe device 40 on the source side of the transformer 18. Likewise, anelbow connector 122 is connected to the line 16 and a load-breakinterface 124 of the switching device 42. A transformer interface 126 ofthe switching device 42 is connected to a connector bushing 128 thatextends through the panel 104 and is coupled to the primary coil 80 toconnect the line 16 to the primary coil 80 through the device 42 on theload side of the transformer 18. A number of positive and negative 120 Vlines 130 and 132 and a neutral line 134 are connected to the secondarycoil 82, extend from the enclosure 100 and provide power along theservice conductor 84, where the number of the lines 130 and 132 dependson the number and type of the loads 86 being serviced by the transformer18.

FIG. 4 is a cross-sectional view of the switching device 40 showing onenon-limiting example merely for illustrative purposes. The device 40includes an outer enclosure 182, the transformer interface 126 and theload-break connector interface 124. The components within the enclosure182 are encapsulated within an insulating medium 190, such as an epoxy,where many of the components are conductors operating at the mediumvoltage potential. A Rogowski coil 188 measures current flow through theswitching device 40. The switching device 40 includes a vacuuminterrupter 196 having a vacuum enclosure 198 defining a vacuum chamber200, an upper fixed terminal 202 extending through the enclosure 198 andinto the chamber 200 and having a contact 204 and a lower movableterminal 206 extending through the enclosure 198 and into the chamber200 and having a contact 208, where a gap 210 is provided between thecontacts 204 and 208 when the vacuum interrupter 196 is open. A bellows212 allows the movable terminal 206 to move without affecting the vacuumintegrity of the chamber 200. The movable terminal 206 is coupled to adrive rod 214 that is coupled to an actuator assembly 216 for openingand closing the vacuum interrupter 196. In this design, the actuatorassembly 216 is insulated and not at the line potential. It is notedthat the details of the vacuum interrupter 196 are merely forillustrative purposes in that other designs will be applicable.Capacitors 220, representing the capacitor 92, provide for voltagesensing and power line communications (PLC).

For most distributed power sources, the inverter 98 would typically be agrid following inverter, well known to those skilled in the art, wherethe source 88 requires a voltage reference from the network 10 tooperate. As the source 88 provides power for the particular load 86, thevoltage on the network 10 provides a voltage reference and allows theload 86 to draw power from the network 10. Therefore, if the load 86requires additional power to function beyond the distributed powergeneration, for example, an air conditioner starting, that power isavailable from the network 10. If power is lost to the network 10 andthe voltage reference is also lost, the distributed power generated bythe source 88 grid following generation should not be able to generateor export power to the network 10, which would be unknown to theutility, thus possibly creating a safety issue. However, there may beconditions at which an unintentional island can form where distributedgeneration continues also creating a safety issue.

In order to address this concern, this disclosure proposes a scheme forpreventing power from being exported to the network 10 after power islost to the network 10 for whatever reason if islanding is detected inthe network 10. When power is lost and after some predetermined timedelay, the switching devices 40 and 76 open to isolate the network 10from the rest of the grid. This prevents the distributed power source 88in the network 10 from exporting power back onto the grid outside of thenetwork 10. When the loss of power is restored, the control unit 90 inthe transformers 18 and 36 compares the measured voltage and the phaseangle at the source side of the transformers 18 and 36 with the measuredvoltage magnitude and phase angle at the load side of the transformers18 and 36 and the measured voltage magnitude and phase angle between theswitching devices in the transformers 18 and 36 to prevent islanding inthe network 10. The voltage magnitude and phase angle are measured forthe case where the voltages on the source side and the load side of thetransformer are the same, but a power distribution source is exportingunsynchronized power into the network 10 at a different phase angle,where a difference in phase angle as a result of different power sourcescould create in a fault in the network 10 if the devices 40 or 76 close.If there is an unacceptable difference between voltage magnitudes or thephase angles, then the switching devices 40 and/or 76 are not closedbecause one or more of the loads serviced by the transformer 18 or 36,or by any of the transformers downstream of the transformers 18 or 36,is injecting power onto the line 16 by a distributed power generationsource 88.

It is noted that in this scheme only the source side switching devicesin the transformers 18 and 76 are opened when loss of power occurs.Alternately, the source side switching devices of all of thetransformers 18-36 can be opened when loss of power is detected, andwhen power is restored, the voltage measurements are performed andcompared as discussed and the switching devices are closed sequentiallystarting at the transformer 18 or 36 along the line 16.

In addition, a scheme can also be provided to prevent the export ofpower from a distributed power generation source 88 onto the network 10during normal operation when the voltage reference point is available,which could otherwise create issues with the utility service. For thisembodiment, the transformers 18-36 use measured current and voltage tomeasure apparent power. For example, if 500 VA of apparent power isinjected in the transformer 18 from the source side and 550 VA ofapparent power is absorbed from the transformer 18 to the load side,then 50 W of net apparent power is being injected by one or more of theloads 86 being serviced by the transformer 18. Because the power isbeing measured by, for example, the Rogowski coil 188 and the capacitors92 and 94, in the switching devices 40 and 42, the control unit 90 cancompare the power coming into the transformer 18 on the line 16 at thesource side with the power flowing out of the transformer 18 on the line16 at the load side. If there is more power flowing out of thetransformer 18 than what is coming into the transformer 18, then thecontrol unit 90 knows that power is being exported onto the serviceconductor 84 by a source 88, which may be a violation of the non-exporttariff. If this happens, the control unit 90 will open the switchingdevices 40 and 42 for a short period of time, such as ten 60 Hz cyclesor less than 2 seconds, until such time that voltage at the source 88 islost. When a loss of voltage occurs the grid following inverter 98 atthe load 86 having the source 88 will disconnect from the network 10because it requires a voltage reference from the network 10, and thereturn to service time, generally 300 seconds, is a requirement for thenon-export tariff. Therefore, when the inverter 98 is disconnected fromthe network 10 for a period of time it no longer has the voltagereference provided by the network 10 and will not be able to producepower during that return to service time. The loads 86 that do not havea source 88 will only see voltage loss for a minimal period of time.Therefore, the load 86 having the source 88 will know that it is inviolation of the non-export tariff as the distributed power generationwill experience trips during periods of export.

Part of the discussion above refers to preventing islanding, i.e.,preventing a load from injecting power onto the network when power islost or otherwise. However, there may be some situations where theutility wants distributed power to be provided to the loads 86 whenpower is lost. For example, if an overhead grid is located in an areathat is susceptible to wildfires and the overhead grid in those areasneeds to be turned off at some point to reduce fire risk, it may bedesirous for underground loop circuits that are less susceptible to fireperils than overhead circuits to remain in service using distributedgeneration. The distributed generation must be sufficient to provide theneeded power to some or all of the loads 86 serviced by the undergroundcircuit.

This type of scheme can be accomplished as follows. If power is lost tothe network 10, then all of the switching devices 40-78 are opened,where the normally open switching device 58 remains open. When thisoccurs, all of the sections serviced by the transformers 18-36 areisolated from each other and any load 86 having a power generationsource 88 serviced by a transformer 18-36 may be able to provide powerfor the other loads serviced by that transformer and possibly additionalloads serviced by other transformers. For this design, the inverter 98may be a grid forming inverter, well known to those skilled in the art,instead of a grid following inverter, which is an inverter thatgenerates its own reference voltage, and therefore does not require areference voltage from the network 10. Alternately, the network 10 canhave a single grid forming inverter positioned somewhere along the line16 that provides the reference voltage, and the power sources 88 wouldnot require grid following inverters.

If a load 86 includes a power generation source 88 and is disconnectedfrom the network 10, its own power generation capability may not besufficient to power loads such as air conditioners that require a highin-rush current. However, if there are multiple loads 86 that have powergeneration sources 88, then the power from the combination of thesesources 88 may be sufficient to power these devices at any one of theloads 86. Therefore, the control unit 90 at each transformer having openswitching devices can control the flow of power among the loads 86serviced by the transformer if there is sufficient power generationamong the loads 86.

If the control unit 90 for the transformer 20 determines that thesources 88 in its section has provided sufficient voltage to support theloads on the conductor 84, it will open its load side switching device46 so that an adjacent source side switching device detects a return ofvoltage and will open to provide power to that transformer's section,which may be combined with voltage already being generated in thatsection from other power generations sources 88. If the control unit 90controlling a load side switching device detects a large enough voltagesag when the source side switching device of the adjacent transformeropens, then it opens since the sources 88 are insufficient to serviceadditional loads. Depending on the configuration of the network 10 andthe amount of power generation that is available, a control unit 90 canalso try and power upstream transformer sections in the same manner, butin this case closing its source side switching device. The switchingdevices will be systematically and relatively slowly closed and voltagedetection will occur because the power generation sources 88 will not beable to immediately provide power to many additional loads and will needtime to ramp up power. In theory, if there is enough power generation inthe network 10 all of the switching devices 42-74 and 78 can eventuallybe closed to provide power to all of the loads 86 in the network 10,where the switching devices 40 and 76 would remain open to isolate thenetwork 10 from the rest of the grid.

When power is restored to the network 10 and is detected at the upstreamside of the switching devices 40 and 76, then a scheme is employed toreturn the network 10 to its normal mode of operation. The transformers18 and 36 send PLC signals to notify the other transformers that theyshould return their power configuration back to normal. After anintentional delay, the control unit 90 in the transformers 18 and 36compares the measured voltage and the phase angle at the source side ofthe transformers 18 and 36 with any measured voltage magnitude and phaseangle at the load side of the transformers 18 and 36 and the measuredvoltage magnitude and phase angle between the switching devices in thetransformers 18 and 36 in the same manner as discussed above to ensurethat the intentional islanding has been shut down, and do not open untilall of the voltages and phase angles are synchronized. In other words,the transformers 18-36 go through a voltage measuring andsynchronization scheme throughout the network 10. When a section is notsynchronized and able to close, the control unit 90 can use a variety ofschemes including PLC transfer trips, intentional delays until asynchronization occurs or load side switching devices to synchronize ortrip grid forming inverters to eliminate unacceptable differences involtage or phase angle before the switching devices are closed.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of thedisclosure as defined in the following claims.

What is claimed is:
 1. A method for allowing islanding in a powerdistribution network, the network including a power line, a plurality oftransformers electrically coupled to and positioned along the powerline, each transformer servicing a section of the network having loadsand including a source side switching device and a load side switchingdevice, wherein source power is provided to both ends of the power lineand wherein one of the switching devices is a normally open switchingdevice, the method comprising: detecting loss of voltage to the network;opening the source side and load side switching devices in all of thetransformers; allowing distributed power generation sources in thesections serviced by the transformers to provide power to the section;closing one or both of the source side switching device and the loadside switching device of a transformer that services a section that haspower generation sources providing excess power; and closing a sourceside switching device or a load side switching device in an adjacenttransformer to the transformer section providing excess power to providepower to the adjacent section.
 2. The method according to claim 1further comprising measuring the voltage at the transformer sectionproviding excess power when the adjacent source side or load sideswitching device is closed and opening the source side switching deviceor the load side switching device in the transformer of the transformersection if the voltage sags.
 3. The method according to claim 2 furthercomprising closing source side and load side switching devices in othertransformers and measuring the voltage in a sequential manner to try andrestore power as many transformer sections as possible.
 4. The methodaccording to claim 1 wherein allowing distributed power generationsources in the sections serviced by the transformers to provide power tothe section includes using a grid forming inverter in power generationsources.
 5. The method according to claim 1 wherein allowing distributedpower generation sources in the sections serviced by the transformers toprovide power to the section includes using a grid forming inverter inthe network.
 6. The method according to claim 1 further comprisingdetecting that voltage has returned to the network, and running a schemeto return the network to normal operation that includes providing powerline communications (PLC) between the transformers and synchronizingvoltage magnitude and phase angles in the network.
 7. The methodaccording to claim 6 wherein running a scheme to return the network tonormal operation includes using PLC transfer trips to trip grid forminginverters.
 8. The method according to claim 1 wherein the source sideand load side switching devices in each transformer are controlled by acommon control unit.
 9. The method according to claim 1 wherein thepower distribution network is an underground residential powerdistribution network.
 10. A method for allowing islanding in anunderground power distribution network, the network including a powerline, a plurality of transformers electrically coupled to and positionedalong the power line, each transformer servicing a section of thenetwork having loads and including a source side switching device and aload side switching device, wherein the source side and load sideswitching devices in each transformer are controlled by a common controlunit, source power is provided to both ends of the power line and one ofthe switching devices is a normally open switching device, the methodcomprising: detecting loss of voltage to the network; opening the sourceside and load side switching devices in all of the transformers;allowing distributed power generation sources in the sections servicedby the transformers to provide power to the section; closing one or bothof the source side switching device and the load side switching deviceof a transformer that services a section that has power generationsources providing excess power; closing a source side switching deviceor a load side switching device in an adjacent transformer to thetransformer section providing excess power to provide power to theadjacent section; detecting that voltage has returned to the network;and running a scheme to return the network to normal operation thatincludes providing power line communications (PLC) between thetransformers and synchronizing voltage magnitudes and phase angles inthe network.
 11. The method according to claim 10 further comprisingmeasuring the voltage at the transformer section providing excess powerwhen the adjacent source side or load side switching device is closedand opening the source side switching device or the load side switchingdevice in the transformer of the transformer section providing excesspower if the voltage sags.
 12. The method according to claim 11 furthercomprising closing source side and load side switching devices in othertransformers and measuring the voltage in a sequential manner to try andpower as many transformer sections as possible.
 13. The method accordingto claim 10 wherein allowing distributed power generation sources in thesections serviced by the transformers to provide power to the sectionincludes using a grid forming inverter in each power generation source.14. The method according to claim 10 wherein allowing distributed powergeneration sources in the sections serviced by the transformers toprovide power to the section includes using a grid forming inverter inthe network.
 15. The method according to claim 10 wherein running ascheme to return the network to normal operation includes using PLCtransfer trips to trip grid forming inverters.
 16. A system for allowingislanding in a power distribution network, the network including a powerline, a plurality of transformers electrically coupled to and positionedalong the power line, each transformer servicing a section of thenetwork having loads and including a source side switching device and aload side switching device, wherein source power is provided to bothends of the power line and wherein one of the switching devices is anormally open switching device, the system comprising: means fordetecting loss of voltage to the network; means for opening the sourceside and load side switching devices in all of the transformers; meansfor allowing distributed power generation sources in the sectionsserviced by the transformers to provide power to the section; means forclosing one or both of the source side switching device and the loadside switching device of a transformer that services a section that haspower generation sources providing excess power; and means for closing asource side switching device or a load side switching device in anadjacent transformer to the transformer section providing excess powerto provide power to the adjacent section.
 17. The system according toclaim 16 further comprising means for measuring the voltage at thetransformer section providing excess power when the adjacent source sideor load side switching device is closed and opening the source sideswitching device or the load side switching device in the transformer ofthe transformer section if the voltage sags.
 18. The system according toclaim 17 further comprising means for closing source side and load sideswitching devices in other transformers and measuring the voltage in asequential manner to try and restore power as many transformer sectionsas possible.
 19. The system according to claim 16 wherein the means forallowing distributed power generation sources in the sections servicedby the transformers to provide power to the section uses a grid forminginverter in power generation sources.
 20. The system according to claim16 wherein the means for allowing distributed power generation sourcesin the sections serviced by the transformers to provide power to thesection uses a grid forming inverter in the network.